The present invention relates to a process for preparing halogen-substituted dibenzyl alcohols from the corresponding halogen-substituted terephthalic acids, to the halogen-substituted dibenzyl alcohols obtainable by this process and to their use for preparing pharmaceutically and agrochemically active compounds.
2,3,5,6-Tetrahalodibenzyl alcohols are known to the person skilled in the art as important intermediates, for example for preparing pharmaceutically or agrochemically active compounds such as insecticides. 2,3,5,6-Tetrafluorodibenzyl alcohol, in particular, is an important intermediate in the preparation of insecticidally active pyrethroids.
The use of 2,3,5,6-tetrafluorodibenzyl alcohol as a starting material in the preparation of household insecticides is described, for example, in EP-A-0 963 976, EP-A-0 959 065, EP-A-0 302 612 and EP-A-0 926 129.
The preparation of 2,3,5,6-tetrafluorodibenzyl alcohol by borohydride reduction from derivatives of tetrafluoroterephthalic acid is described in GB-A-2,127,013. Here, the reduction of tetrafluoroterephthalic acid chloride is carried out using sodium borohydride in diglyme at a temperature of 20xc2x0 C. giving the tetrafluorodibenzyl alcohol in a yield of only 64.4%. Nothing is said about the purity.
Furthermore, WO-A-00/68173 describes the synthesis of tetrafluorodibenzyl alcohols by catalytic reduction of the corresponding tetrafluoroterephthalonitrile via the intermediates tetrafluoroterephthalaldehyde tetraalkyl acetal and tetrafluoroterephthalaldehyde. However, in the case of the preparation of 2,3,5,6-tetrafluorodibenzyl alcohol, a purity of only 93.4% is obtained. The total yield, starting from 2,3,5,6-tetrafluoroterephthalonitrile, is 48% (see Examples 15 and 18 in this document). Also described is the synthesis of 2,3,5,6-tetrafluorobenzyl alcohol by the same process. The yields of about 80% and the purities of  greater than 99% described in a number of examples in this document are considerably higher than those in the synthesis of the corresponding dibenzyl alcohol.
The synthesis of 2,3,5,6-tetrafluorodibenzyl alcohol is also described in the as yet unpublished German patent application with the reference 100 03 320, by hydrogenation of 2,3,5,6-tetrafluoroterephthalonitrile to 2,3,5,6-tetrafluorodibenzylamine and subsequent diazotization and heating of the amine. The 2,3,5,6-tetrafluorodibenzyl alcohol obtained in this manner has a purity of 74.5%; however, for certain applications, this is still not entirely satisfactory. According to the example, the total yield starting from 2,3,5,6-tetrafluoroterephthalonitrile is 52.1%.
WO-A-00/68173 describes a process in which, comparably to the process of the German patent application with the reference 100 03 320, 2,3,5,6-tetrafluorobenzyl alcohol is prepared from 2,3,5,6-tetrafluorobenzonitrile. The purity of 99.3% and the yield of 85.1% achieved here are considerably higher than those achieved in the synthesis of the corresponding 2,3,5,6-tetrafluorodibenzyl alcohol.
DE-A-37 14 602 describes a process for preparing 2,3,5,6-tetrafluorobenzyl alcohols which may be substituted in the 4-position by a methyl radical, by reacting 2,3,5,6-tetrafluorobenzoic acids which may be substituted in the 4-position by a methyl radical with sodium borohydride and then with an alkylating agent in the presence of a diluent at temperatures between xe2x88x9220xc2x0 C. and +150xc2x0 C. The reaction of the tetrafluorobenzoic acids is preferably carried out at 0-30xc2x0 C. and the subsequent reaction with the alkylating agent is preferably carried out at 0-100xc2x0 C., where from 0.5 to 0.9 mol of an alkylating agent are employed per mole of starting material. The process affords the 2,3,5,6-tetrafluorobenzyl alcohol in a yield of 96.7% and a purity of 98%.
The hydrolysis with sulfuric acid of 2,3,5,6-tetrafluoroterephthalonitrile to 2,3,5,6-tetrafluoroterephthalic acid is described in EP-A-0 749 409.
As is known to the person skilled in the art, the use of insecticides, in particular household insecticides, is preceded by intensive toxicological studies. As a consequence, it is desirable to be able to employ the active compounds in a defined form which is as pure as possible. In the synthesis of insecticidesxe2x80x94in particular when 2,3,5,6-tetrafluorodibenzyl alcohol is usedxe2x80x94the byproducts which are substituted asymmetrically in the benzyl position, such as 1-(benzyl alcohol)-4-benzylamines, 1-benzaldehyde-4-benzyl alcohols and 1-(benzoic acid)-4-benzyl alcohols are disruptive, since these compounds can pass through the entire synthesis and even into the end product, and may therefore remain in the active compound and can exert undesirable toxic effects.
All of the three processes for preparing 2,3,5,6-tetrafluorodibenzyl alcohols mentioned above (GB-A-2,127,013, WO-A-00/68173 and DE-A-100 03 320) have in common the disadvantage that, starting from tetrafluoroterephthalonitrile, the reaction proceeds via more than one intermediate. An additional, decisive disadvantage is the fact that in all other processes used both for preparing monobenzyl alcohols and dibenzyl alcohols, the dibenzyl compound is in each case obtained in a considerably poorer yield and purity. The conclusion that had to be drawn from these facts was that apparently the dibenzyl function of the desired product and the asymmetric intermediates formed during a synthesis greatly affect both purity and yield.
Accordingly, it was an object of the present invention to provide a process which allows the preparation of halogen-substituted dibenzyl alcohols starting from the corresponding halogen-substituted terephthalic acids, with high yields and purities and in particular substantially free of compounds substituted asymmetrically in the 1- and 4-positions.
The invention relates to a process for preparing a halogen-substituted dibenzyl alcohol of the general formula (I): 
wherein the radicals X are identical or different and are hydrogen, fluorine, chlorine or bromine and at least one X is fluorine, chlorine or bromine. The process comprises reacting (1) a halogen-substituted terephthalic acid of the general formula (II) 
wherein the radicals X are as defined for the general formula (I) with (2) sodium borohydride and then with (3) an alkylating agent or sulfuric acid or an alkyl-sulfonic acid or an arylsulfonic acid, wherein the sulfuric acid or the alkyl-sulfonic acid or the arylsulfonic acid comprises at most 5% by volume of water and the sodium borohydride reacts in an organic solvent at a temperature ranging from about 0 to about 150xc2x0 C.
The invention also relates to a halogen-substituted dibenzyl alcohol of the general formula (I): 
wherein the radicals X are identical or different and are hydrogen, fluorine, chlorine or bromine and at least one X is fluorine, chlorine or bromine, wherein the alcohol is obtained by a process comprising reacting (1) a halogen-substituted terephthalic acid of the general formula (II) 
wherein the radicals X are as defined for the general formula (I) with (2) sodium borohydride and then with (3) an alkylating agent or sulfuric acid or an alkylsulfonic acid or an arylsulfonic acid,
wherein the sulfuric acid or the alkylsulfonic acid or the arylsulfonic acid comprise at most 5% by volume of water and the sodium borohydride reacts in an organic solvent at a temperature ranging from 0 to about 150xc2x0 C.
The invention also relates to a halogen-substituted dibenzyl alcohol of the general formula (I) 
wherein the radicals X are identical or different and are hydrogen, fluorine, chlorine or bromine and at least one X is fluorine, chlorine or bromine, wherein the alcohol comprises at most 1.0% of compounds which are substituted asymmetrically in the 1- and 4-positions. These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims.
The invention provides a process for preparing halogen-substituted dibenzyl alcohols of the general formula (I) 
where the radicals X are identical or different and are hydrogen, fluorine, chlorine or bromine and at least one X is fluorine, chlorine or bromine, by reacting (1) halogen-substituted terephthalic acids of the general formula (II) 
where the radicals X are as defined for the general formula (I), with (2) sodium borohydride and then with an alkylating agent or sulfuric acid or alkyl- or arylsulfonic acids, where the sulfuric acid or the alkyl- or arylsulfonic acids comprise at most about 5% by volume of water and the reaction with sodium borohydride is carried out in an organic solvent at a temperature in the range of from about 0 to about 150xc2x0 C.
The borohydride reduction according to the invention of the halogen-substituted terephthalic acids of the general formula (II) under the conditions mentioned and the subsequent reaction with alkylating agent, sulfuric acid or alkyl- or arylsulfonic acid succeeds with an unexpectedly high purity and simultaneously high yield. Products of the formula (I) obtained are substantially free or, in the preferred case, completely free of compounds which are substituted asymmetrically in the 1- and 4-positions, in particular of halogen-substituted 1-(benzyl alcohol)-4-benzylamines, halogen-substituted 1-benzaldehyde-4-benzyl alcohols and halogen-substituted 1-(benzoic acid)-4-benzyl alcohols.
Starting materials for the process according to the invention are halogen-substituted terephthalic acids of the general formula (II) where the radicals X are identical or different and are hydrogen, fluorine, chlorine or bromine and at least one X is fluorine, chlorine or bromine. Preference is given to using 2,3,5,6-tetrafluoro- and 2,3,5,6-tetrachloroterephthalic acid, i.e. all four substituents X are fluorine or chlorine. The use of terephthalic acids having mixed substituents, where two X are hydrogen and in each case one X is chlorine or fluorine has also been found to be useful.
The person skilled in the art is familiar with the preparation of such halogen-substituted terephthalic acids of the general formula (I). The halogenated terephthalic acids of the general formula (II) are preferably prepared by acid hydrolysis of halogenated terephthalonitrile of the general formula (III) 
where the radicals X are as defined for the general formula (III).
This acid hydrolysis can be carried out in a simple manner and with high purities. If this group is chosen for preparing the halogen-substituted terephthalic acid, the halogenated dibenzyl alcohol which is finally obtained by the process according to the invention is particularly pure. This variant has been found to be particularly useful for preparing 2,3,5,6-tetrafluoroterephthalic acid from 2,3,5,6-tetrafluoroterephthalonitrile. In the process according to the invention, the reaction with sodium borohydride is carried out at a temperature in the range of from about 0 to about 150xc2x0 C., preferably from about 20 to about 80xc2x0 C. and in particular from about 40 to about 65xc2x0 C.
The reaction according to the invention with sodium borohydride is carried out in an organic solvent. Preference is given to using an aprotic polar organic solvent, such as, for example, diethyl ether, diisopropyl ether, diisobutyl ether, methyl t-butyl ether, 1,2-dimethoxyethane, diglycol dimethyl ether, tetrahydrofuran and 1,4-dioxane. Particular preference is given to 1,2-dimethoxyethane. In a particular embodiment, sodium borohydride is initially charged and 2,3,5,6-tetrafluoroterephthalic acid is added as a solution in this solvent.
In cases where an alkylating agent is added after the borohydride reduction, it has been found to be useful to employ from about 2.0 to about 2.5 mol, preferably from about 2.1 to about 2.4 mol, particularly preferably from about 2.1 to about 2.3 mol of sodium borohydride per mole of halogenated terephthalic acid of the general formula (II) for the reduction.
In contrast, in cases where sulfuric acid or an alkyl- or arylsulfonic acid is employed after the borohydride reduction, it has been found to be useful to employ from about 2.0 to about 4.0 mol and preferably from about 2.5 to about 3.0 mol of sodium borohydride per mole of halogenated terephthalic acid of the general formula (II) for the reduction.
Suitable for use as alkylating agents are, for example, alkyl halides, dialkyl sulfates and sulfonic esters of the general formulae (IV), (V) and (VI):
Rxe2x80x94Hal ROxe2x80x94SO2xe2x80x94OR2 Rxe2x80x94SO2xe2x80x94OR2 (IV) (V) (VI) 
where R and R2 independently of one another are methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, tert-butyl, phenyl or tolyl and Hal is Cl, Br or I.
Here, from about 1.0 to about 2.0 mol and preferably from about 1.4 to about 2.0 mol of the alkylating agent are employed per mole of the halogenated terephthalic acid of the general formula (II) originally employed.
In the process according to the invention, it is furthermore also possible to employ, instead of an alkylating agent, sulfuric acid or an alkyl- or arylsulfonic acid which comprises at most about 5% by volume of water. Suitable for use as alkylsulfonic acid are, for example, methyl-, ethyl-, propyl- or butylsulfonic acid. Phenyl- or tolylsulfonic acid have been found to be suitable arylsulfonic acids. In this case, from about 0.5 to about 1.0 mol of sulfuric acid or from about 1.0 to about 2.0 mol of an alkyl- or arylsulfonic acid are usually employed per mole of the halogenated terephthalic acid of the general formula (II) originally employed, where the sulfuric acid or alkyl- or arylsulfonic acid comprises in each case at most 5% by volume of water.
The alkylating agent, too, can be added diluted with an inert solvent. For this purpose, it is possible to use the polar aprotic organic solvents which can be employed for the borohydride reduction. Other inert solvents suitable for diluting the alkylating agent are C5-C30-, preferably C6-C12-alkanes, C5-C30-, preferably C6-C12-cycloalkanes, C6-C20-, preferably C6-C12-aromatics, C2-C10-esters and C2-C30-ethers. Preference is given to using hexane, heptane, petroleum ether, cyclohexane, decalin, benzene, toluene, the xylenes, chlorobenzene, dichlorobenzene, trichlorobenzene, ethyl acetate, butyl acetate, diethyl ether, tetrahydrofuran and diphenyl ether. Particular preference is given to toluene and xylene.
In the process according to the invention, the reaction with the alkylating agent, the sulfuric acid or the alkyl- or arylsulfonic acid is carried out, for example, at temperatures in the range from about 0 to about 150xc2x0 C., preferably from about 30 to about 80xc2x0 C., particularly preferably from about 40 to about 70xc2x0 C. and especially from about 50 to about 65xc2x0 C.
The process according to the invention is usually carried out at atmospheric pressure or at an elevated pressure of up to about 10 bar.
Work-up and isolation of the reaction product can be carried out by customary methods. Preferably, the reaction mixture is diluted with a solvent which is virtually water-immiscible. Also advantageous is the option of solvent exchange by distillation. Particularly advantageous is the use of the diluent of the alkylating agent for this purpose. It is then possible to hydrolyze using water or an aqueous acid. The aqueous phase can be extracted, if appropriate at elevated temperature, and the product can then be crystallized from the extraction agent, for example after concentration or again after solvent exchange, to reduce the solubility of the product in the solvent, and be isolated by filtration.
In the work-up steps, such as, for example, the distillation, it has been found to be useful to lower the pressure to about 0.001 bar.
By the simple process steps of a borohydride reduction and reaction with alkylating agent, sulfuric acid, alkyl- or arylsulfonic acid, the process according to the invention affords the desired halogenated dibenzyl alcohol of the general formula (I) in a purity of more than about 98.5%, with at the same time excellent high yields.
Accordingly, the invention furthermore provides the halogen-substituted dibenzyl alcohols of the general formula (I) which can be obtained by the process according to the invention.
These halogen-substituted dibenzyl alcohols of the general formula (I) obtainable by the process according to the invention have a purity of at least about 98.5%. They comprise at most about 1.0%, preferably at most about 0.1%, particularly preferably at most about 0.01% of compounds substituted asymmetrically in the 1- and 4-positions, in particular halogen-substituted 1-(benzyl alcohol)-4-benzylamines, 1-benzaldehyde-4-benzyl alcohols and 1-(benzoic acid)-4-benzyl alcohols, and, in particular, they are completely free of these compounds. The proportion required to make up 100 is due to other impurities which enter the system via the starting materials in the initial steps leading to the halogen-substituted terephthalic acids and are carried over into the end product.
These particularly pure halogen-substituted dibenzyl alcohols are suitable for use as intermediates in the preparation of pharmaceutically and agrochemically active compounds.
Here, the person skilled in the art can refer to the extensive literature for example on the preparation of household insecticides, for example EP-A-0 963 976, EP-A-0 959 065, EP-A-0 302 612 and EP-A-0 926 129.
The invention is further described in the following illustrative examples in which all parts and percentages are by weight unless otherwise indicated.